Artful Science: Peering into Ancient Pigments

The chemistry skills and know-how needed to extract pigment from a plant or insect and create a solid dye date back some 4,000 years to ancient Egypt, new research concludes. Previous analysis had only confirmed such techniques to about 1200 B.C.

Analyzing pigment has been an important way of identifying and studying artwork for decades. Chemical profiles of colored material can tell a detailed story about when, where, how, and sometimes even by whom a piece of art was made.

But the older the remnant, the fewer fragments of color typically remain. And organic dyes—such as those derived from plants and insects—have required larger samples for analysis than inorganic colors extracted from the most ancient, mineral-based pigments. Taking a sizable sample of pigment from a painting or sculpture for analysis is "absolutely out of the question," says Marco Leona, a scientist at The Metropolitan Museum of Art (the Met) in New York City; doing so would destroy too much of the work. So, until now, many ancient works have kept their secrets locked away in natural pigments.

But by improving on a tried-and-true laser-based microanalysis technique, Leona, head of the Met's Scientific Research Department, has been able to analyze priceless pieces of pigment from extremely old cultural products without damaging their integrity.

The process—a version of surface-enhanced resonance Raman scattering (SERRS)—uses silver nanoparticles to absorb miniscule amounts of dye molecules. In doing so, it overcomes two key obstacles that had prevented the compounds from being studied in the past: The silver nanoparticles enhance the reading of the otherwise dilute dyes; they also prevent the otherwise fluorescent substances from reflecting too much light when a laser is shined on them.

The findings, published online today in the Proceedings of the National Academy of Sciences, reveal some of the oldest known uses of various natural dyes, including the discovery of madder (red, plant-based) dye on a leather Egyptian quiver remnant (2124 to 1981 B.C.)—thought to be the earliest example of such pigment use.

The results were obtained by analyzing microscopic samples, some less than 25 micrometers across. "We were surprised that it worked with…samples [that small]," Leona says. With previously available methods, he notes, "we would have had to remove a visible chunk."

The importance of studying organic dyes in addition to traditional mineral-based pigment is clear to Leona. "They correspond to major industries, commerce and technology," he says of the dye advances. The organic pigments can also be studied to identify forgeries and match similar works of art.

New analysis also helped to confirm trade of nonnative dyes from south Asia to Europe. Although documents had noted the use of a red insect-based dye called lac by A.D. 1222, it had yet to be found on any European art that early. However, after analyzing an A.D. 1150 to 1200 piece, Virgin and Child in Majesty (the Morgan Madonna) from Auvergne, France, Leona found it to contain the dye, which had most likely passed through north Africa from India.

He and the research team will continue testing even older items in the museum's collection (where "the [new] technique has been almost routine," he says) to see if the earliest dates for these organic dyes can be pushed back even further. But he is also anxious to use the technique on more modern work. With the explosion of organic chemistry in the 19th century, he says, all the natural dyes are supplanted by manufactured dyes, which were promptly patented. Such specific date information for the pigments will help scholars date art and "get a sense of the global commerce," tracking these compounds as they spread around the globe.

The knowledge will also bring new power to the art industry, Leona says. "By knowing what's in them, we have a better chance of conserving them."